3.6 - Enthalpy Changes for Solids and Solutions

Hess’ Law

The total enthalpy change for a chemical reaction is independent of the route where the reaction takes place, provided the initial and final conditions are the same

Exothermic reactions

• Gives out heat

• Moree chemical energy present in the reactants that the products

• Forming bonds

• -ve value

Endothermic reaction

• Takes in heat

• More chemical energy in the products than the reactants

• Bond breaking

• +ve value

Enthalpy change of formation

The enthalpy change when 1 mole of a substance/compound is formed from its constituent elements in their standard states, under standard conditions

Enthalpy change of combustion

The enthalpy chnage when 1 mole of a substance reacts completely with oxugen under standard conditions of 298K and 101kPa

Enthalpy of lattice dissociation/breaking

The enthalpy change when 1 mole of an ionic substance/compound fully dissociates into its gaseous ions. Always +ve

Enthalpy of lattice association/forming

The enthalpy change when 1 mole of an ionic substance/compound is formed from its gaseous ions. Always -ve

Enthalpy of solution

The enthalpy change when 1 mole of a solid ionic substance/compound is dissolved in water under standard conditions

Enthalpy of hydration

The enthalpy change when 1 mole of aqueous ions are formed from their gaseous ions/1 mole of gaseous ions is associated with water molecules. Always -ve

Enthalpy change of atomisation

The enthalpy change when 1 mole of an element in its standard state is atomised to produce 1 mole of gaseous atoms. Always +ve

Enthalpy change of ionisation energy

METAL IONS ONLY. The enthalpy change when 1 mole of electrons is removed from 1 mole of gaseous atoms/ions. Always +ve

Enthalpy change of electron affinity

NON-METAL IONS ONLY. The enthalpy change when 1 mole of electrons are added to 1 mole of gaseous atoms/ions. +ve or -ve

Which enthalpy change is used to determine stability of an ionic compound

Enthalpy of formation

Enthalpy of formation and stability of ionic compounds

-ve = energy given out = more stable than the elements from which it was formed = more stable

+ve = energy taken in = unstable compared with the elements from which it is formed = more unstable, less stable

-ve enthalpy of formation and stability

More stable

+ve enthalpy of formation and stability

More unstable, less stable

Process of an ionic substance dissolving

• Ionic lattice breaks down into gaseous ions (dissociation/breaking)

• Gaseous ions are hydrated (hydration)

Exothermic enthalpy change

-ve

Endothermic enthalpy change sign

+ve

General enthalpy change equation

products-reactants

remember to include stoichiometry in the calculation

Possible combinations for Born Haber cycles

Group 1 + 7

1+6

2+7

2+6

Process group 1 atoms undergo in Born Haber cycles

1st ionisation to form 1+ ions

Process group 2 atoms undergo in Born Haber cycles

1st and 2nd ionisation to form 2+ ions

Process group 7 atoms undergo in Born Haber cycles

1st electron affinity to form 1- ions

Process group 6 atoms undergo in Born Haber cycles

1st and 2nd electron affinity to form 2- ions

How to draw a Born Haber cycle

1. Place the ionic product on a line at the base of the diagram

2. Place a line with the elements above it - energy change between the two is enthalpy change of formation

3. Add the constituent ions in the gaseous state to a top line on the right hand side - enthalpy change between solid and gaseous ions is lattice formation

4. Add the stages of atomisation, ionisation (both upwards) and electron affinity (downwards) to the diagram

5. Sum of all clockwise enthalpies=sum of all anti-clockwise enthalpies

Rule to use when finding enthalpy changes from Born Haber

Sum of clockwise enthalpies=sum of all anti-clockwise

Fromula for enthalpy of solution

Lattice dissociation + sum of hydration

Draw a diagram of a a general Born Haber cycle

Enthalpy of solution and solubility

-ve = likely to be soluble

+ve = likely be insoluble

Factors that influence enthalpy of dissociation and association

• Stronger the attractions between ions, greater enthalpy values

• Factors that influence strength of association

  • Charge ion; greater —> stronger

  • Ionic radius; smaller —> smaller

Sign for lattice dissociation

Always +ve

Sign for lattice association

Always -ve

Sign for enthalpy of solution

Either positive or negative, depends on ions

Sign for enthalpy for hydration

Always -ve

Factors that affect enthalpy of hydration sign

Greater charge and smaller ionic radius —> more exothermic value —> more negative value

Enthalpy of formation sign

-ve or +ve

Enthalpy of atomisation sign

Always +ve

Enthalpy change of ionisation energy sign

Always +ve

Enthalpy change of electron affinity sign

+ve or -ve

Enthalpy of formation equation for Born Haber cycle

Sum of all other energy terms